This invention relates to communication methods and systems and more particularly a communication method and system providing privacy by virtue of its ability to change, on a coded, predetermined basis the time against frequency relationship of the transmissions on a continuous basis.
The "chirp" type of signal is one which continuously changes frequency with time according to some present relationship. In present "normal" use this signal is transmitted at one location and received at either the same location or at a different location for high frequency ionospheric propagation research purposes. However, the prior art does not include the utilization of a "chirp" type signal for the purpose of privacy in radio communication.
This invention solves the problem of generating a communications signal for use in the high frequency, very high frequency, or ultra high frequency range, with the following characteristics: all transmissions have a high degree of privacy or security for both pulsed and voiced communications circuits without the requirement for critical encryption techniques. Transmissions in the high frequency range where ionospheric propagation must be depended upon may be improved by the capability for determination of the optimum frequencies for operation. The detectability of the communications signals by unfriendly forces is low. The susceptibility of the communications signal to jamming and/or intentional interference is low. Finally, the susceptibility of the communications signals to position location by conventional direction finding equipment is greatly reduced.
This invention will be useful to the Air Force and others for providing secure, reliable, high quality communications from point-to-point. It will be useful for both ground-based, air, and shipborne uses. It will also be useful for command and control aircraft which must communicate over longer distances using the high frequency band. It will be useful to SAC for high frequency secure communications to manned bombers over long distances. When used with certain present forms of secure communications techniques, this invention will improve the security of the signal. Since this invention is jam-proof and virtually detection-proof, it will be useful for high reliability circuits. Since the signal is very difficult, if not impossible with the present state-of-the-art, to locate by direction finding techniques, clandestine operations by agents would be feasible.